This invention relates to nuclear well logging and, more particularly, to methods for enhancing the vertical resolution of nuclear logging instruments.
Nuclear well logging tools usually employ detector systems such as gas filled tubes or scintillation detectors for counting either neutrons or gamma rays. These devices are event sensitive in the sense that an event such as passage of a gamma ray or neutron through the detector generates a discrete output or count of its occurrence. The events may occur at high frequency or low frequency depending on the source of the radiation being detected and other factors. But, in any case, the counts of the detector occur in a random, unpredictable time sequence and are summed, accumulated or integrated over a time or depth interval. Thus, the data for a particular time or depth interval is represented by a number of counts having a statistical uncertainty associated with it.
The nuclear logging tool outputs are typically sampled on a depth basis in most commercial well logging systems. Material intervening between the nuclear source and the detector of course alters the vertical response of the device. The system response function which is a characteristic of a particular logging tool geometry, collimation, source detector spacing, shielding, etc. also impacts the vertical response of the device. Generally, the vertical response has been thought of as being limited primarily by the source-detector spacing in the past. Movement of the logging tool through the borehole as measurements are made also blurs response to adjacent formations and further integrates or averages physical properties of the formation sought to be measured such as gamma ray activity, formation density and formation porosity.